The present invention relates to a switching amplifier for a high voltage transmitter.
FRG Offenlegungsschrift [Laid-open Application] No. 1 816 530 describes a low frequency, high output power amplifier that is suitable for the modulation of the end stage of high frequency broadcasting transmitters. The amplifier consists of a plurality of controlled components acting together with a shared working resistor, preferably with uniform steps. Switching of the components from one state to another by modification of the control variable for the individual components is effected in time sequence in such a manner that the sum of the output variables is essentially an analog reproduction of the control variables. The controlled components are in series with the working resistor and are located at voltage sources with low internal resistance and rectifiers that allow passage of the effective current being inserted in the current paths from the terminals of the voltage sources to respective connection points between two of the controlled elements. The magnitude of the activating voltage is equal to that of the output voltage, whereby a linear relationship between the controllable voltage and the output voltage is to be achieved for the respective partial voltages. Such a design not only presents the disadvantage of very high control voltage but also makes enormously high demands on the limiting frequency of the components. Meeting such demands is barely possible when controlled semiconductors are used in this amplifier as modulation amplifiers for powerful radio transmitters (about 500 kW). Such a circuit also has the disadvantage that the voltage demanded by the rectifiers is very high.
It is considered more advantageous to arrange the partial voltage sources in the currently usual form of cascades of voltage sources in which the voltage demanded by the rectifiers serving as free-wheeling diodes is uniform. Such arrangements used as low frequency amplifiers is the basis of FRG Offenlegungsschrift DE-OS No. 30 44 956 Al and European Patent Application No. 0 066 904 Al.
In an amplifier described in Offenlegungsschrift 30 44 956 Al, cascade-connected partial voltage sources are turned on or off by a clocked or non-clocked A/D converter that is controlled by an analog signal. To ensure that an output signal which is an analog to the input signal is generated, rather than a gradated signal with coarse gradations, an amplifier arrangement is connected in series to the partial voltage sources and is controlled by an error signal generated by the difference between the input signal and output signal of the cascade connection of partial voltage sources. The use of such a circuit as a low frequency amplifier for the high powers cited above presents great disadvantages since the level sensing of the A/D converter is affected by delays, and the turning on and off of the partial voltage sources is affected by switching time errors and by fluctuations in the voltage supply for the partial voltage sources that are still not taken into account, thus causing the signal correction amplifier circuit to be overloaded for the transmission of a low-distortion output signal as required for modulation of the high-frequency output stage of radio transmitters.
The last-cited Offenlegungsschrift also discloses an amplifier circuit with binary staggering of partial voltage sources. Because of the limited electrical strength of semiconductors, such an arrangement is suitable only for low-power amplifiers (below 10 kW), and in no case as a semiconductor modulation amplifier for high-power radio transmitters.
The switching amplifier disclosed in European Patent Application No. 0 066 904 Al presents an arrangement of partial voltage sources similar to that of the last-cited Offenlegungsschrift. Here, too, a clocked A/D converter is used to control the partial voltage sources. The correction of the gradated, coarse-gradation output voltage signal is sought by obtaining fine gradation through division of an amplifier step into substeps whose output voltages are binarily weighted. It is further mentioned that it is further possible to correct the gradated, coarse - gradation output voltage signal with the aid of a delayed control signal.
In the switching amplifier described, nonlinear distortions may occur which, particularly at higher frequencies, are greater than permitted under international norms governing such quality requirements for radio transmitters. Because of the switching time error in semiconductor switches, improving gradation characteristics in the described manner is possible only for low frequencies. To permit improvement of the output voltage via a delayed control signal, the clock frequency for the A/D converter must not be too high. But this is in basic contradiction to the purpose of high frequency transmission. Because of the unavoidable switching errors in semiconductor switches, the output voltage in the case of semiconductor switches is affected by amplitude errors, particularly at high frequencies, these errors being, for system-related reasons, asymmetric to the zero voltage value of the analog alternating voltage. Asymmetry in the output signal results in a high distortion factor in the output signal.